This invention relates generally to a combustion system and, more specifically, to a combustion system that comprises a primary reaction zone; and a secondary reaction zone, which includes an injector for injecting a fluid into a stream of combustion products generated within the primary reaction zone.
Generally, some combustions systems produce mechanical torque by combusting a fuel and air mixture. This process yields by-products including undesired emissions. Some combustion systems form an essential part of a turbomachine, such as, but not limiting of, a gas turbine. The primary emissions are oxides of nitrogen (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHC). Oxidation of molecular nitrogen in air breathing machines, such as, but not liming of, gas turbines, is highly dependent upon the maximum temperature in the combustion system reaction zone and the residence time for the reactants at the maximum temperatures reached within the combustor. The level of thermal NOx formation is minimized by maintaining the reaction zone temperature below the level at which thermal NOx is formed or by maintaining an extremely short residence time at high temperature such that there is insufficient time for the NOx formation reactions to progress.
One method of controlling the temperature of the reaction zone below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. U.S. Pat. No. 4,292,801, dated October 1981, describes a dual stage-dual mode low NOx combustor based on lean premixed combustion technology for a gas turbine application. U.S. Pat. No. 5,259,184, dated November 1993, describes a dry low NOx single stage dual mode combustor construction for a gas turbine.
The thermal mass of the excess air present in the reaction zone of a lean premixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where thermal NOx formation is reduced. However for some turbomachines, the required temperature of the generate combustion products entering the first stage turbine nozzle, at some load, requires operation at peak gas temperature in the reaction zone. Peak operation typically exceeds the thermal NOx formation threshold temperature resulting in significant NOx formation even though the fuel and air are in a lean premixed form. Therefore, there is a desire to obtain combustor exit temperatures high enough to meet the requirements of those turbomachines, without forming a significant amount of thermal NOx.
Lean premixed combustion of hydrocarbon fuels in air is widely used throughout the turbomachine industry for reducing emissions levels; in particular, thermal NOx emissions levels for gas turbine combustors. Lean direct injection (LDI) of hydrocarbon fuel and air is also an effective method for reducing NOx emission levels.
Some LDI systems require an active cooling system. The active cooling system may comprise an atomizing-air compressor, manifold, tubing, and other costly structure. An active cooling system may be considered a parasitic load on the output of the turbomachine; and consumes a significant amount of energy and requires operational costs.
Some LDI systems may not uniformly distribute the injected fuel and air mixture, which may lead to local high-NOx regions and cause hot streaks on some turbine and combustion hardware. Here, the position of the injectors may be fixed and not allow for adjustment.
There may be a desire for an improved method of operating an LDI system. The method should incorporate a passive cooling system and not require an active cooling system. The method should also allow for adjusting a position of the fluid exiting the injector.